We are continuing the high resolution X-ray structural study of the yeast enzyme pyrophosphatase. In the near future we plan to complete the processing of diffraction photographs of the native protein and two isomorphous heavy atom derivatives. This will be following by the calculation of the electron density map of the structure and subsequently the construction of a molecular model using a Vector General 3400 graphics computer. These studies will be followed by structural studies of enzyme-substrate complexes of pyrophosphatase. In this way we hope to be able to elucidate the catalytic mechanism of the enzyme.